An image sensor conventionally comprises a matrix of pixels each comprising a photodiode and a control circuit suitable for providing an output signal representative of the level of illumination received by its photodiode. Such a sensor makes it possible to acquire a discretized and digitized image of a scene (or digital image).
FIG. 1 is a partial sectional view of an example of a front-illuminated image sensor. More precisely, FIG. 1 illustrates two pixels of a pixel matrix of the image sensor. Each pixel includes a photoconversion area 2 consisting of a layer of semiconductor material. The image sensor optionally includes memory areas 4 formed in the layer of semiconductor material. The photoconversion areas 2 are separated from each other and the memory areas 4 by deep isolation trenches 6 or DTI (“Deep Trench Isolation”), which makes it possible to electrically and optically isolate them. Various transistors, not represented, e.g., selection, transfer and initialization transistors, may also be formed in and on the layer of semiconductor material.
The image sensor further includes an insulating layer 8, e.g., made of silicon oxide, covering the layer of semiconductor material. Interconnection levels are buried in the insulating layer 8. The interconnection levels are formed of metallizations 10 connected to each other by conductive vias (not shown). The interconnection levels make it possible to connect different elements formed in and/or on the layer of semiconductor material, e.g., transistors (not shown), to each other or to the surface of the structure. The metallizations 10 are placed in the insulating layer 8 in such a way that the portions of the insulating layer 8 opposite each photoconversion area 2 are free of metallizations 10, which allows the passage of light rays up to the photoconversion area 2. Metallizations may further be formed above memory areas 4 so as to prevent light rays from reaching these areas.
In the example represented in FIG. 1, the structure is covered with another layer 12 of insulator. The layer 12 is, for example, an anti-reflection layer. Lenses 14 are arranged on the layer 12. A lens 14 is placed opposite each photoconversion area 2 so as to focus the light rays towards the corresponding photoconversion area 2. Additional layers may be placed between the lenses 14 and photoconversion areas 2, e.g., color filters.
The features of the lenses 14 located opposite the photoconversion areas 2, e.g., their focal distance, are determined in such a way that substantially all of the light rays the wavelengths of which belong to a selected wavelength range reach the photoconversion area 2.
In the example in FIG. 1, this wavelength range corresponds to the visible domain, i.e. wavelengths between 380 and 780 nm.
Defects commonly appear on the digital image when using sensors such as those currently manufactured. These defects appear notably when the scene comprises very bright objects, e.g., the sun. Often a deformation of the image is found, e.g., the formation of halos (“flare”) around the sun, or the presence of ghost images, e.g., multiple suns.
It would be desirable to provide an image sensor remedying all or part of these drawbacks.